This invention relates generally to battery chargers, and more particularly, to controlling the charging of batteries with battery chargers.
Battery chargers are used in many different applications to charge batteries, such as batteries for portable and mobile devices (e.g., land mobile radios). These battery chargers range from wall connected units to car connected units. Different battery chargers also may provide different rates of charging and different charge levels based on, for example, the type of battery.
Rechargeable batteries may be constructed of different materials and provided in different cell configurations. For nickel based rechargeable batteries, such as nickel-cadmium (nicad or NiCd) or nickel metal hydride (NiMH), different methods are known to determine when to terminate charging of the battery. These methods determine when the battery is fully charged within a tolerance range. One known method for terminating charging is to monitor the temperature gradient (ΔT/dt) of a battery pack. Once a battery approaches a full charge, the battery can no longer absorb the charging energy and converts the energy to heat. The charging of the battery is terminated when the slope of the change in temperature of the battery reaches a certain level, for example, one degree Celsius per minute.
Known methods for monitoring the change in slope of the battery temperature may not always adequately indicate that a battery is fully charged. For example, when a cool battery is charged in a warm or elevated temperature environment, for example, in a car using a car charger, the environmental heat can warm the battery pack. This increase in the temperature gradient due to the environmental temperature absorbed by the battery results in early termination of the charge prior to a full charge state. Thus, known chargers using these methods of charging may prematurely terminate charging of a battery due to environmental effects and not based on charging effects.